Chronic back problems cause pain and disability for a large segment of the population. In many cases, the chronic back problems are caused by intervertebral disc disease and loss of stability of the intervertebral joint. Stabilization and/or arthrodesis of the intervertebral joint can reduce the pain and debilitating affects associated with disc disease.
Spinal stabilization systems and procedures have been developed to stabilize diseased intervertebral joints and, in some cases, to fuse the vertebrae that are adjacent to the diseased joint space. Most fusion techniques include removing some or all of the diseased disc material from the affected joint, stabilizing the joint and inserting a bone graft or other material to facilitate bony fusion of the vertebrae.
One type of spinal stabilization system includes screws and connecting rods which can be used for stabilizing many spinal conditions including, for example, degenerative disc disease, scoliosis, spondylolithisis and spinal stenosis. Examples of such systems are disclosed in U.S. Pat. Nos. 6,010,503; 5,946,760; 5,863,293; 4,653,481, etc., the entire disclosures of which are incorporated herein by reference. In these systems, a bone screw (e.g., pedicle screw) is typically anchored into each vertebral body to be stabilized and a rigid connecting rod mounted to the screws to fix the vertebrae in a particular relative position. Generally, these systems provide posterior column support but lack anterior column support.
Another type of spinal stabilization system includes interbody implants such as disclosed in, for example, U.S. Pat. Nos. 5,458,638; 5,489,307; 5,055,104; 5,026,373; 5,015,247; 4,961,740; 4,877,020; 4,743,256; and 4,501,269, the entire disclosures of which are incorporated herein by reference. Some of these implants are bone, some are solid titanium or similar non-bone implant material and some are hollow implants that provide for inclusion of a bone graft or other suitable material to facilitate bony union of the vertebrae.
Interbody implants can be inserted into the disc space through an anterior, posterior or lateral approach. When two implants are used, the implants are typically positioned parallel to one another on either side of a sagittal plane passing through the midline of the vertebral bodies. In some systems, the implants are inserted into a bore formed between adjacent vertebral bodies in the cortical endplates and can extend into the cancellous bone deep to the cortical endplates. Implant size is typically selected such that the implants force the vertebrae apart to cause tensing of the vertebral annulus and other soft tissue structures surrounding the joint space. Tensing the soft tissues surrounding the joint space results in the vertebrae exerting compressive forces on the implant to maintain the implant in place.
However, in some cases, the compressive forces exerted on the implant may cause undesired pressure induced changes to the bone adjacent the implant. Pressure induced changes can lead to reduced joint stability, increased fusion time and increased chance of subsidence or implant migration.
Accordingly, there is a continuing need for improved vertebral stabilizing devices and methods. The present invention is directed to addressing these needs.